1. Field of the Invention
The present invention relates to a fuel cell and a fuel cell stack formed by stacking a plurality of the fuel cells. Each fuel cell is formed by sandwiching an electrolyte electrode assembly between metal separators. The electrolyte electrode assembly includes a pair of electrodes, and an electrolyte interposed between the electrodes.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a polymer ion exchange membrane as a solid polymer electrolyte membrane. The solid polymer electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly. Each of the anode and the cathode is made of electrode catalyst and porous carbon. The membrane electrode assembly is sandwiched between separators (bipolar plates) to form the fuel cell. In use, generally, a predetermined number of the fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas (reactant gas) such as a gas chiefly containing hydrogen (hereinafter also referred to as the hydrogen-containing gas) is supplied to the anode. An oxidizing gas (reactant gas) such as a gas chiefly containing oxygen (hereinafter also referred to as the oxygen-containing gas) is supplied to the cathode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electrical energy.
In the fuel cell, for example, a metal plate is used for fabricating the separator. The strength of the metal separator is high in comparison with a carbon separator, and the metal plate is suitable for fabricating a thin separator. The metal separator with corrugated surfaces including protrusions and grooves which define reactant gas flow fields having the desired shape is fabricated by press forming in order to reduce the thickness of the metal separator, and to achieve reduction in the overall size and weight of the fuel cell.
In the press forming process of the metal separator, bending forming or roll forming is performed. Therefore, residual stress exists in the metal separator. After press forming, undesirable deformation such as warpage or distortion may occur in the metal separator. Therefore, the uniform surface pressure distribution in the electrode surface or the sealing surface may not be achieved at the time of stacking the metal separator and the other components of the fuel cell. Thus, the power generation performance or the sealing performance is lowered undesirably.
As the conventional technique which is aimed to address the problem, for example, a fuel cell as disclosed in Japanese Laid-Open Patent Publication No. 2002-175818 is known. In the conventional technique, as shown in FIG. 11, a separator 1 is fabricated by press forming using one metal plate. The separator 1 includes a current collector region 2 and a marginal region 3 around the current collector region 2. The current collector region 2 has a corrugated surface including grooves forming a gas flow field and protrusions which contact an electrode assembly. A rib 4 is formed along the entire circumference of the marginal region 3.
As described above, since the rib 4 is formed in the marginal region 3 of the separator 1, in comparison with the case in which the marginal region 3 has a flat surface, the rigidity of the marginal region 3 is increased greatly. Even if the separator 1 is a thin plate, deformation such as warpage after press forming is effectively suppressed.
Internal manifold structure is widely adopted in the fuel cell. In the internal manifold structure, fluid supply passages and fluid discharge passages extend though separators in the stacking direction. The fuel gas, the oxygen-containing gas, and the coolant are supplied from the respective fluid supply passages into the fuel gas flow field, the oxygen-containing gas flow field, and the coolant flow field, and then, the fuel gas, the oxygen-containing gas, and the coolant are discharged into the respective fuel gas discharge passages.
Therefore, for example, as the gas supply passages and the gas discharge passages, six fluid passages extend though the separators in the stacking direction. If the separator is a metal thin plate, the rigidity of the portion around the fluid passage is very low. Therefore, deformation occurs easily in the position near the fluid passage. The uniform surface pressure may not be achieved at the time of stacking the separator and the other components of the fuel cell.